3.8 Undefining and Redefining Macros

If a macro ceases to be useful, it may be undefined with the
`#undef' directive. `#undef' takes a single argument, the
name of the macro to undefine. You use the bare macro name, even if the
macro is function-like. It is an error if anything appears on the line
after the macro name. `#undef' has no effect if the name is not a
macro.

#define FOO 4
x = FOO; ==> x = 4;
#undef FOO
x = FOO; ==> x = FOO;

Once a macro has been undefined, that identifier may be redefined
as a macro by a subsequent `#define' directive. The new definition
need not have any resemblance to the old definition.

However, if an identifier which is currently a macro is redefined, then
the new definition must be effectively the same as the old one.
Two macro definitions are effectively the same if:

Both are the same type of macro (object- or function-like).

All the tokens of the replacement list are the same.

If there are any parameters, they are the same.

Whitespace appears in the same places in both. It need not be
exactly the same amount of whitespace, though. Remember that comments
count as whitespace.

If a macro is redefined with a definition that is not effectively the
same as the old one, the preprocessor issues a warning and changes the
macro to use the new definition. If the new definition is effectively
the same, the redefinition is silently ignored. This allows, for
instance, two different headers to define a common macro. The
preprocessor will only complain if the definitions do not match.